1. Technical Field of the Invention
The present invention relates to an engine starting apparatus which is able to engage a pinion of a starter with a freewheeling ring gear in the course of an engine stop process to restart the engine.
2. Related Art
Providing vehicles with an idle stop system is an important approach to reducing CO2 as one of the countermeasures against global warming. The idle stop system is a system, for example, that stops fuel injection to an engine to automatically stop the engine when the vehicle is stopped at an intersection due to a stop signal or in pause due to traffic jam or the like.
Conventional idle stop systems have been configured to automatically stop an engine after the vehicle has been fully stopped. In order to further improve the effect of reducing CO2, it is effective to elongate an engine stop period. Elongating the engine stop period may be specifically achieved by a system that stops an engine before the vehicle speed runs out (i.e. during the deceleration preceding the vehicle stop), converting from the conventional systems that stop the engine after the vehicle has been fully stopped. It is expected that such a system that elongates the engine stop period may significantly improve the effect of reducing CO2, compared to the conventionally used idle stop systems.
However, this system raises an issue incurred in a potential restart of an engine after the engine has entered an engine stop process. Specifically, in conventional starters, the pinion of the starter cannot be engaged with the ring gear of the engine unless the engine is fully stopped. This means that, in the case where an engine is restarted using a conventional starter, the engine cannot be restarted from the point when the engine has entered the engine stop process up to the point when the engine is completely stopped. There may be a situation, for example, that the traffic light at an intersection is red and the vehicle is decelerated accordingly followed by the output of a stop command to allow the engine to enter the engine stop process, and that, then, the traffic light quickly turns green. In such a situation, conventional starters cannot immediately restart the engine, which may cause trouble to the following vehicle and impose a psychological burden on the user. Accordingly, in order to use the idle stop function while the vehicle is decelerating, it is essential to enable restart of the engine when the engine is in the engine stop process.
In order to realize restart during the engine stop process, the pinion of the starter is required to be in engagement with a ring gear in rotation. A technique as a method of realizing such a restart is disclosed in WO2007/101770. Specifically, this patent document discloses a method of restarting an engine using a starting device that includes a first RPM detecting means that detects the number of revolutions of a ring gear, a second RPM detecting means that detects the number of revolutions of the pinion of a starter or the number of revolutions of a motor, and a motor revolution control driver that controls the number of revolutions of the motor. In this starting device, the number of revolutions of the pinion is controlled by the motor revolution control driver based on the number of revolutions detected by the first and second RPM detecting means, for synchronization with the number of revolutions of the ring gear. As a result, the pinion is engaged with the ring gear.
The method disclosed in WO2007/101770 (the method of synchronizing the number of revolutions of a pinion with that of a ring gear to establish engagement between the gears) is an ideal method in the case where gears distanced from each other are brought into engagement with each other. However, this method has a large problem of requiring a motor revolution control driver that controls the number of revolutions of a motor. Generally, an MOS transistor as a control element is used as a motor revolution control driver to perform voltage control (e.g., pulse width control, so-called PWM control). However, starter motors have a low voltage (usually 12 V) in spite of having a large output. Therefore, this necessitates the use of an MOS transistor having a large current capacity exceeding 500 A and thus greatly raises the cost as a result.
In addition, achieving synchronization between the numbers of revolutions of a pinion and a ring gear may require feedback control of the numbers of revolutions. As a result, a long time will be taken for the synchronization. Therefore, in many cases, there is a concern that synchronization is unlikely to be completed during the very short time in which the engine speed is decreasing.